When you want to protect the little girl's smile, but you're the bad guy. (Or a meat puppet of the bad guy, as it were.)
And so the mystery of Julian's and Darius' motives is cleared up; Julian wants to kill Pandora to save her from Darius' machinations (or at least he pretends to want her dead), and Darius wants to save Pandora from Julian so he can proceed with his plan, but sharing a body means one can't keep Pandora away from the other. That is... one hilariously twisted antagonist relationship.
Not that it wasn't obvious that Darius only wanted Pandora so he could open her Box, but now the ambiguity has been dropped with all the subtlety of a Looney Tunes anvil; you can't hoist a "Big Villain" flag much higher than calling someone a "thing".